A request that wraps multiple other requests, allowing multiple requests to be treated as a single request.

Note that Request.isCancelled() and Request.cancel() apply to all requests contained by the composite request. In other
words, cancelling this request immediately marks all contained requests as cancelled. However, cancelling any request in the
request has the effect of cancelling all other requests in the composite, including the composite. (This is implemented by
having all Request objects in the composite share the same cancelled flag object.)